(a) Field of the Invention
The present invention relates to a power source apparatus used in electronic equipment such as an electronic typewriter.
(b) Related Background Art
In a power source apparatus which uses batteries whose power source voltage cannot be kept constant and which drives, e.g., a motor in electronic equipment, an unstable voltage is boosted or dropped to a given constant voltage level, and the motor or the like is driven by the stabilized voltage.
FIG. 6 shows an example of a conventional power source apparatus. A voltage 3 supplied from batteries 1 is output through a voltage transformer 71 as stable voltages V.sub.H.72 and V.sub.L.73. This apparatus exemplifies a circuit for driving a 4-phase stepping motor 9. Windings 9.1 to 9.4 of the motor 9 are driven by transistors 5, 10, 11, 12, and 13.
When a control signal 6 is set at "L" (low) level, the transistor 5 is turned on. In addition, when a control signal 14 is set at "H" (high) level, the transistor 10 is turned on. A current flows from the output V.sub.H.72 of the voltage transformer 71 to the transistor 10 through the transistor 5 and the winding 9.1. Similarly, currents flow or do not flow through the windings 9.2, 9.3, and 9.4 of the motor 9 in response to control signals 15, 16, and 17, respectively.
As a result, the motor 9 is rotated in accordance with a sequence of the control signals 14 to 17.
When the control signal 6 is set at "H" level to disable the transistor 5, and a terminal 74 is connected to the output V.sub.H.72, as indicated by a dotted line in FIG. 6, a current flows from the output V.sub.H.72 to the winding of the motor 9 through a resistor 8 and a diode 7. In this case, an amount of current flowing through the motor 9 is smaller by a voltage drop through the resistor 8 as compared with the former case. The operations of the former and latter cases will be briefly described with reference to FIG. 7.
FIG. 7 shows a current waveform of one phase when the motor 9 is driven by 2-phase excitation. The motor 9 is driven while the transistor 5 is kept ON. Referring to FIG. 7, for example, a current starts to flow through the winding 9.1 while the transistor 10 is kept ON. The transistor 10 is then turned off, and the current flowing though the winding 9.1 becomes zero. Thereafter, the transistor 10 is turned on to cause the current to flow through the winding 9.1 again. When the transistor 5 is turned off during the ON state of the transistor 10, a current I.sub.L smaller than that during motor rotation flows through the winding 9.1. The current I.sub.L is adjusted by the resistance of the resistor 8. A small current is generally supplied to the motor to hold the phase position so as not to offset it from the proper position of the corresponding phase during the stop state of the stepping motor. This small current value is determined by a load of an apparatus connected to a motor. Referring to FIG. 6 when the terminal 74 is connected to the output V.sub.H.72, the power consumption of the resistor 8 is increased. When the voltage V.sub.L.73 satisfying V.sub.H &gt;V.sub.L is generated by the voltage transformer and connected to the terminal 74, the power consumption of the resistor 8 can be reduced, and power consumption of the batteries 1 can also be reduced. This method can be employed to cause the small current I.sub.L to flow. However, the voltage transformer 71 is undesirably complicated, thus increasing cost.
In the above prior art, the small current I.sub.L of the motor is derived from the output V.sub.H.72 or V.sub.L.73 through the voltage transformer 71. However, the voltage transformer 71 also consumes power, and transformation efficiency cannot be given as 100% but is reduced to about 70 to 90%. Furthermore, when the power source comprises batteries, this transformation efficiency is very important to determine the battery service life. In particular, when the power source voltage is gradually decreased upon long-term use of the batteries, the efficiency is degraded accordingly. Therefore the service life of the batteries is rapidly shortened.
When the voltage V.sub.H.72 is connected to the terminal 74 as in the conventional case, the power consumption of the resistor 8 is increased due to heat dissipation. However, when the voltage output level is made higher by using the voltage V.sub.L.73 or the like, the voltage transformer is complicated only to result in high cost